An electrical generator produces alternating current from a motive power typically produced by the rotation of a prime mover such as a gas turbine, a water turbine or a wind turbine for example. When the electrical generator is used as a power production generator to be connected to a large power grid, the frequency of the produced alternating current must match the utility frequency of the power grid. The power production generator will need to be synchronized with the power grid before it is connected.
In typical electrical generators, the rotor is mechanically coupled to the prime mover such that when the speed of the prime mover varies, so does the frequency of the generated electric current. In cases where the speed of the prime mover may vary, such as with typical wind turbines, the speed of the rotor needs to be regulated or the frequency variation of the produced electric current to be corrected.
One solution is to use power electronics to correct the frequency of the produced electric current. A high-power rectifier is used to rectify the produced variable-frequency alternating current to provide direct current. Direct current is then converted back to alternating current with the required frequency using a high-power inverter (AC to DC to AC conversion). Power electronics is energy consuming and decreases the overall efficiency of the electric generator.
The rotation speed of the turbine may also be regulated by adjusting the opening of the supply valve in the case of a water turbine or by adjusting the angle of attack of the blades in the case of a wind turbine. However, the angle of attach often cannot be adjusted with a sufficient time response in cases of a gust of wind.